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Dense Wavelength Division Multiplexing Total Attenuation Curve Fig-10 Attenuation curve Other Factors that affect the Fiber transmission DISPERSION POLARISATION EFFECT OTHER NONLINEAR EFFECTS 3.32 Light Source & Detectors Light emitters and light detectors are active devices at opposite ends of an optical transmission system. Light sources, or light emitters, are transmit-side devices that convert electrical signals to light pulses. The process of this conversion, or modulation, can be accomplished by externally modulating a continuous wave of light or by using a device that can generate modulated light directly. Light detectors perform the opposite function of light emitters. They are receive-side opto-electronic devices that convert light pulses into electrical signals. Light Emitters—LEDs and Lasers Light emitting devices used in optical transmission must be compact, monochromatic, stable and long-lasting. Division Of Computer Engineering, SOE 19
Dense Wavelength Division Multiplexing Two general types of light emitting devices are used in optical transmission, light-emitting diodes (LEDs) and laser diodes, or semiconductor lasers. LEDs are relatively slow devices, suitable for use at speeds of less than 1 Gbps, they exhibit a relatively wide spectrum width, and they transmit light in a relatively wide cone. These inexpensive devices are often used in multimode fiber communications. Fig-11 Emitter & Photodiode operation Requirements for lasers include precise wavelength, narrow spectrum width, sufficient power, and control of chirp (the change in frequency of a signal over time). Semiconductor lasers satisfy nicely the first three requirements. Light Detectors On the receive end, it is necessary to recover the signals transmitted at different wavelengths on the fiber. Because photo detectors are by nature wideband devices, the optical signals are demultiplexed before reaching the detector. Two types of photo detectors are widely deployed, the positive-intrinsic-negative (PIN) photodiode and the avalanche photodiode (APD). PIN photodiodes work on principles similar to, but in the reverse of, LEDs. That is, light is absorbed rather than emitted, and photons are converted to electrons in a 1:1 relationship. APDs are similar devices to PIN photodiodes, but provide gain through an amplification process: One photon acting on the device releases many electrons. PIN photodiodes have many advantages, including low cost and reliability, but APDs have higher receive sensitivity and accuracy. However, APDs are more expensive than PIN photodiodes, they can have very high current requirements and they are temperature sensitive. Division Of Computer Engineering, SOE 20
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